1. Field of the Invention
The present invention relates to a flat aluminum electrolytic capacitor and a method of manufacturing the same. More particularly, the present invention relates to a flat aluminum electrolytic capacitor, comprising a capacitor element having a stacked structure or a capacitor element having wound structure, that is encased in a hermetically sealed flexible casing, and a method of manufacturing the same.
2. Description of the Related Art
Electrolytic capacitors have conventionally been manufactured by encasing a capacitor element, comprising two electrode foils (an anode foil and a cathode foil) and a separator interposed therebetween which are wound in a roll, in a cylindrical metal casing, and stopping the openings of the metal casing with a terminal board having lead terminals.
As electronic apparatuses and optical apparatuses such as cameras have become smaller in profile and in thickness, the conventional aluminum electrolytic capacitor of cylindrical configuration is viewed as too large and it takes too much space on a circuit board compared to other electronic components. Although capacitors may be made flat instead of the cylindrical construction, there is a dimensional limitation in manufacturing the flat capacitor thus generating an obstacle to making apparatuses smaller in profile and in thickness. Accordingly, there are demands for capacitors having configurations different from those of the prior art, in order to fit in the component mounting space of the smaller electronic apparatuses. To meet these demands, an electrolytic capacitor having a hermetically sealed structure of composite films has been proposed as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 55-143024 (Patent Document 1) and Japanese Unexamined Patent Publication (Kokai) No. 58-178517 (Patent Document 2). Since the electrolytic capacitor having such a hermetically sealed structure of composite films can be made to have an external configuration in accordance to the shape of the capacitor element, an electrolytic capacitor of a desired shape, besides the conventional cylindrical shape, such as rectangular, sheet and oval configurations can be manufactured, thus greatly contributing to a decrease in the profile and thickness of the electronic apparatuses.
The electrolytic capacitor disclosed in Japanese Unexamined Patent Publication (Kokai) No. 55-143024 (Patent Document 1) is manufactured by covering a capacitor element with a metal-resin composite laminate film and hermetically sealing the capacitor element by thermo compression bonding of the composite laminate film at the periphery.
The electrolytic capacitor disclosed in Japanese Unexamined Patent Publication (Kokai) No. 58-178517 (Patent Document 2) is manufactured by covering a capacitor element with a metal film and a composite film comprising a metal film laminated with plastic film on both sides thereof, and hermetically sealing the capacitor element by thermo compression bonding of the composite laminate film.
The aluminum electrolytic capacitor of the prior art comprises the capacitor element that is manufactured by disposing an anode foil and a cathode foil, opposing each other via a sheet of separator paper interposed therebetween. The capacitor element is then impregnated with an electrolytic solution and is encased in a metal casing. The casing is then stopped and sealed at the openings thereof. To stabilize the performance of the capacitor of such a structure, an appropriate level of voltage is applied across the capacitor in a relatively high temperature atmosphere, so as to carry out a anodization, or the so-called aging treatment. In the aging treatment, damage caused, during the capacitor forming process, to an aluminum oxide film formed on the anode foil and the cathode foil are remedied.
While hydrogen gas is generated through a reaction between the electrolytic solution and the foil electrodes during the aging treatment, the capacitor element of the prior art is encased in a metal casing having enough strength to resist the pressure of hydrogen gas generated during the aging treatment so as to maintain the external configuration thereof, because the opening of the casing is sealed by calking with a strong force. An attempt has also be made where the casing is sealed by means of sealant made of rubber through which hydrogen gas is discharged to the outside (Japanese Unexamined Patent Publication (Kokai) No. 57-1219: Patent Document 3). With this construction, however, what can be discharged through diffusion via the rubber sealant is the small amount of hydrogen gas generated during use of the capacitor after it has been manufactured, at most. Hydrogen gas generated rapidly during the aging treatment cannot be discharged with this construction. Even during normal operation, the gas pressure rises so as to cause deformation of the capacitor and/or degradation of performance at times when hydrogen gas is generated at a high rate which cannot be accommodated by the rubber sealant.
As a result, it has been common to use a metal casing that is strong but is not easy to process into a desired shape, in order to ensure performance and strength.
In the meantime, in order to allow the use of the capacitors in wider range of products, methods such as as forming the capacitor in a desired shape have been studied so as to meet the conditions of the product where it is to be used with emphasis placed on the flexible processing capability, and using a flexible material or a thin sheet of less strength in forming the casing for the purpose of reducing the capacitor size. However, as such a casing has insufficient strength, troubles such as swelling or rupture of the capacitor, deformation of the capacitor element and degradation of the characteristics of the capacitor, occur when the inside pressure of the casing rises as hydrogen gas is generated during the aging treatment. Also, during normal use, hydrogen gas is generated and accumulates through the reaction between the aluminum foil electrodes and the electrolytic solution, the inside pressure of the casing rises and causes the capacitor to swell.
In the case of aluminum electrolytic capacitor of flat configuration or stacked structure, the foil electrode is more liable to stress and damage the dielectric film compared to the conventional aluminum electrolytic capacitor of cylindrical shape. As a consequence, there have been such problems as a significant amount of hydrogen gas is generated during the aging treatment, thereby causing the casing to swell, deform or rupture, the electrolytic solution to leak, the capacitor element to be deformed by the hydrogen gas, and the characteristics of the capacitor to deteriorate. Also during normal use, hydrogen gas is generated and accumulates through the reaction between the aluminum foil electrodes and the electrolytic solution, and therefore the inside pressure of the casing rises causing the capacitor to swell.
The present invention has been devised in order to overcome the drawbacks of the prior art technology described above and make full use of the space, in the final product, available for the capacitor. For this purpose, the present invention provides a method of manufacturing a flat aluminum electrolytic capacitor, wherein a flat capacitor employing a flat or stacked capacitor element that is encased in a casing made of such a material which can be formed into a desired and compact shape (for example, a flexible and strong film or a laminated sheet) or a flexible casing that accommodates the shape of the capacitor element with the casing being sealed, while hydrogen gas generated during the manufacturing process is discharged to the outside so that the capacitor remains compact without swelling. The present invention also provides a flat aluminum electrolytic capacitor having a small size where hydrogen gas generated during the manufacturing process is dispersed to the outside and capable of absorbing hydrogen gas generated during use so as to maintain the shape without deformation, that is manufactured by the method described above.